Endoscopic third ventriculostomy
Third ventriculostomy, an endoscopic neurosurgery procedure designed to treat obstructive hydrocephalus.
Hydrocephalus is the buildup of cerebrospinal fluid in the ventricles of the brain.
Hydrocephalus is divided into two flavors:
- obstructive hydrocephalus: where there is a block to CSF drainage from the ventricles, most commonly at the cerebral aqueduct in the midbrain (this is the narrowest pipe that water has to traverse, thus is most likely to occlude). Typical etiologies include congenital and tumor-related.
- communicating hydrocephalus: where water gets out of the brain just fine, but then has nowhere to go, i.e. the absorptive mechanisms are deranged. Typical etiologies include post-meningitis, post-subarachnoid hemorrhage, and"normal pressure hydrocephalus."
Here are two CT scans, one with normal -sized ventricles and the other with ventricles enlarged by hydrocephalus. Notice how the "mantle" of brain thins out over the dilated ventricles. Neurons don't like this, and intellectual decline often results.
Hydrocephalus was a real scourge of mankind until very recently: children would get born, their ventricles would blow up with cerebrospinal fluid (CSF) , their heads would begin to expand from this pressure (remember, the skull takes its signal for growth from the underlying brain), and the child would plummet in functional status but remain alive as a tiny little body attached to a head the size of a watermelon. Yes, it really was that bad.
Treatment of hydrocephalus got a major boost with the invention of the valved CSF shunt system, allowing a plastic tube to drain the ventricles through a one-way valve which only permitted water flow above a certain pressure gradient, into the venous or peritoneal spaces. These shunt systems, typically ventriculoperitoneal (VP), have become the mainstay of hydrocephalus treatment in the past 40 years.
But every silver lining has its cloud. At least 10% of VP shunts need to be revised because of obstruction, infection, growth of the child, or other reasons. If you have a shunt, you need to take antibiotics before you get your teeth worked on, for fear of infecting the shunt system. So although loads better than nothing, shunts have their limitations and frustrations too.
Enter imagination, therefore. Wouldn't it be neat, neurosurgeons thought, if we could take cases where there was a block of CSF flow, obstructive hydrocephalus, and simply create a hole from the ventricles inside the brain to the subarachnoid space outside the brain? That way CSF produced in the ventricles could flow out this man-made hole, and get absorbed in the usual fashion. And fenestrate they did. Most of these holes and the hardware they put into them wound up closing off in an annoyingly regular fashion, and the operations that it took to make the holes began to look like too much surgery for the results after VP shunting came on the scene. VP shunts took the lead, and a period of relative dormancy for hole-makers ensued.
This field was reignited, however, when many neurosurgeons, including Alan Cohen, began looking around the ventricular system with endoscopes. With an access hole around 12 mm diameter (a standard burr-hole), one could introduce a perfectly useful endoscope into the ventricles, look around, and actually perform surgery. Wow.
In fact, placing a burr-hole in a very standard place on the head, just anterior to the coronal suture and 2-3 cm off the midline, would allow you to look straight through the foramen of Munro into the third ventricle! Double wow.
And, in looking at the floor of the third ventricle in people with obstructive hydrocephalus, it becomes apparent that this floor is often very thin, begging the surgeon to fenestrate it. Dr. Cohen and others gave in to the temptation, with remarkably frequent and durable success. Thus, the genesis (or renaissance, selon goût) of endoscopic third ventriculostomy for the treatment of obstructive hydrocephalus.
And thus, Third Ventriculostomy
Endoscopic third ventriculostomy is a procedure which, in selected patients with obstructive hydrocephalus, allows egress of CSF from the ventricles to the subarachnoid space. This can decompress the ventricles and allow normal intracranial pressures and brain growth. And you get all of this benefit with no permanently implanted hardware.
The rest of this page will deal with patient selection, techniques, outcomes, and pointers for further info relating to endoscopic third ventriculostomies.
Patient selection for endoscopic third ventriculostomy
This really is the crux of the matter.
This procedure doesn't work all the time, and you want to maximize your patient's probability of success with each procedure offered.
Features which increase a patient's probability of success include:
- obstructive hydrocephalus
- age > 1 year
- relatively recent onset of the obstruction
- no meningitis or subarachnoid hemorrhage history
- enlarged ventricles
- normal ventricular anatomy
||Axial flow-gated MR scan showing absence of flow through the aqueduct in the midbrain
In adults with recent-onset aqueductal stenosis (e.g. patients with posterior fossa metastatic disease), one can get success rates approaching 80%. Other groups generally have lower probabilities of success, more like 50-60% (some say 20%). However, the procedure is relatively low-risk and does not preclude the placement of a VP shunt. If third ventriculostomy fails, a VP shunt can be placed. Patients should be informed of the procedure and its success rate.
Thus patient selection depends on your attitude: if you look at a 50% success rate as half of the people becoming shunt-independent with all its benefits, you do 3V's a lot. If you look at this as a 50% failure rate with these patients needing a second procedure, you are more selective with your 3V's. Often the best way to decide is to inform the patient/family and let them mull it over. Some people are willing to take risk A and some people are willing to take risk B, and both are satisfied with the outcomes of their respective choices.
Technique of endoscopic third ventriculostomy
NOTE: This section is a general description of the technique of third ventriculostomy. Do not use this information as a guide to performing third ventriculostomy. If you wish to perform third ventriculostomies, you must obtain special training in neuro-endoscopy. (For our foreign friends: yes, idiots really try to do this stuff and then sue you when they fail...)
OK, now that that's out of the way, let's see how we do this operation. Third ventriculostomy is divided into several stages. We will illustrate each in turn. The stages of this operation are:
- finding and entering the foramen of Munro
- Inspecting the floor of the third ventricle
- Perforating the floor of the third ventricle
- Enlarging this perforation
- Inspecting the prepontine cistern
The endoscope is inserted through a burrhole approximately 2-3 cm lateral to the midline and just anterior to the coronal suture. A disposable plastic sheath facilitates insertion and manipulation of the endoscope.
Finding and entering the foramen of Munro
On entering the ventricle, one first identifies the formanen of Munro. The foramen of Munro is located where the thalamostriate and septal veins, as well as the choroid plexus of the lateral ventricle, converge.
The endoscope is then passed carefully through the foramen of Munro into the third ventricle and the anatomy is defined.
going thru foramen of Munro, 1.1M QuickTime movie (temporarilly offline)
||Choroid plexus at 4:00, septal vein above it, thalamostriate vein at 8:00, and fornix comprising anterior-superior border of the foramen of Munro. Brought to you by your friendly endoscopist.
Inspecting the floor of the third ventricle.
No, it doesn't come annotated like this. But this is what you look for. Often the basilar artery can be seen through the translucent floor of the third ventricle. This is reassuring, because one wishes to perforate the third ventricular floor without perforating the basilar artery. For ten points, write a short essay detailing why.
Perforating the floor of the third ventricle
The ventriculostomy hole is placed anterior to the basilar artery, usually being made with a blunt instrument such as a guide wire, closed forceps, or the endoscope itself.
||Instrument indenting floor of third ventricle just prior to perforation. Avoid that basilar artery!
Enlarging this perforation
The hole is then enlarged to approximately 5 mm diameter with a Fogarty balloon catherter. Yep, the same catheter the vascular surgeons use.
||Fogarty catheter about to be withdrawn thru the hole in the third ventricular floor. Surgeon's heart usually stops around this point.
Inspecting the prepontine cistern
One then inspects the ventriculostomy hole to ensure free communication of CSF into the subarachnoid space, then the endoscope is withdrawn. Note basilar artery, short perforating arteries to the pons, superior cerebellar artery, posterior cerebral artery, pons, seat of the soul.
Some surgeons leave an external ventriculostomy for a few days, some don't.
||The Promised Land: one must see this anatomy to be sure both third ventricle and arachnoid have been perforated.
||Sagittal flow-gated MR showing a jet of CSF shooting between the prepontine cistern and the third ventricle. Gives your endoscopist warm fuzzies.
And that's it, except for the followup. Third ventriculostomy patients should be followed up just like shunted hydrocephalus patients.
Ballpark success figures include:
less than 1 y of age,
2 or more y of age,
over 2 y of age,
adult with recent onset of
Mean time to failure runs approximately 2-6 weeks, but failures have occurred years out from third ventriculostomy.
Success in third ventriculostomy can be a bit harder to measure. If the velocity of head growth normalizes, the ventricles decrease in size, the fontanelle becomes soft and pulsatile, one is usually willing to declare victory.
Marginal cases are more difficult to manage, and third ventriculostomists will usually err on the side of shunting a marginal result, because it is imperative to make sure that a child's hydrocephalus is treated adequately in the
first years of life.
Third ventriculostomy failure
But what, you ask, when a third ventriculostomy fails?
Well, the fallback position is always to convert to a VP shunt. Dr. Christian Saint Rose in Paris, however, inspected his failed third ventrics (with MR and cine MR imaging), and found that many of them failed because a membrane grew over the hole that he had made. He then re-perforated the membrane and waited. Mirabile dictu, seven out of nine of these re-third ventriculostomies remained open! (JNS, vol. 90, p. 448, 1999)
So there is an alternative to VP shunting in third ventriculostomy failure, namely reperforation of the original hole. The author has begun doing this and finds that the membrane is usually thin and translucent, making the reperforation relatively straightforward.
For More Info:
Third ventriculostomy in general (not exhaustive, so don't get mad at me, guys...):
- Jones RFC, Kwok BCT, Stening WA, & Vonau M: The current status of endoscopic third ventriculostomy in the management of non-communicating hydrocephalus. Minim. Invas. Neurosurg, v. 37, pp. 28-36, 1994.
- Perneczky A, Tschabitscher M, & Resch KDM: Endoscopic Anatomy for Neurosurgery. New York: Thieme Medical Publishers, Inc., 1993.
Suggested Reading: AANS Neuroendoscopy Course Pamphletby Alan Cohen: 708 692-9500 (ask for their continuing education division)